1. Field of the Invention
This invention relates to a system for detecting particulate contaminants associated with ceramic sheets during processing in the manufacture of integrated circuit modules.
2. Prior Art
In the manufacture of integrated circuit modules, unfired ceramic sheets (green sheets) are subjected to a variety of processing techniques, such as blanking, punching, screening with a conductive paste and stacking into multilayer modules. The green sheets are generally flexible and soft until fired. They have a propensity to carry with them particulate contaminants, typically ceramic debris, which tend to adhere to their soft surfaces. These contaminants are carried with the green sheets from station to station since handling is generally by vacuum pick-up or Bernouilli techniques. Thus, at those stations where debris is likely to be created, such as in blanking and punching of the green sheet material, it is difficult to eliminate all contaminants when the sheets are moved to the next processing station.
The existence of these contaminants is especially severe during processing to screen a conductive paste pattern on the green sheet. During this process step, a thin mask is placed over the green sheet wafer for the purpose of screening a highly complex and fine pattern of conductive lines. The presence of such contaminants has a twofold effect. First, they cause dents in the mask during screening resulting in the destruction of the mask and poor screened pattern dimensional control. In the formation of integrated circuit modules, layers of green sheets are stacked to define a multi-layer ceramic module. Alignment from layer to layer is crucial and the existence of a dent caused by a particle destroys the conductive alignment in the mask which is used to screen conductive paste on that respective green sheet. This expensive mask must be discarded.
Secondly, the presence of a contaminant inhibits effective screening of the conductive pattern. In the absence of a test to determine whether particles are present, screening takes place and it is only in subsequent quality control steps that the accuracy of the screening procedure is determined. Should the screening be defective, the sheets are generally unusable. Thus, in addition to destroying the mask, defective green sheets are produced.
Within the prior art, there is general recognition of the problem of damage which occurs to screening masks by particles associated with green ceramic sheets. For example, IBM TDB, Vol. 23, No. 4, pp. 1333-1334 aligns green sheets onto an alignment fixture and then utilizes the input transfer head, typically a vacuum device to press the green sheet, that is, flatten it, on the alignment fixture. This techniqu e tends to embed any particles and flatten any icicle-like projections which may exist on the surface of the green sheet into the green sheet body per se. Thus, there is no attempt to sense the presence of contaminants but rather this technique proceeds on the assumption that such are present in all green sheets and simply performs a flattening operation. This technique, therefore, embeds contaminants into the green sheet. This leads to the propensity of making it difficult to lift the green sheet off the alignment fixture since it is pressed into position. Finally, this technique adds an increment of processing time into the overall system since the transfer head which is normally used to simply move green sheets from one station to another must remain at a particular station for the purpose of performing the flattening operation.
IBM TDB, Vol. 12, No. 11, page 1897, April, 1970 disclosed an optical inspection technique for detecting surface protrusions on semiconductor wafers. A blank mask is lowered onto the surface of the wafer and a vacuum is then applied. If protrusions are present, strains will be set up in the mask on the wafer surface. When viewed through an analyzer using polarized light, such protrusions are displayed as bright spots on a uniform field. This manual inspection technique is therefore premised on operator judgment such as the number and size of the observed spots and allows for the rejection of wafers considered harmful to the screening masks without actually employing those screening masks or damaging any test mask. Such an optical inspection technique is usable in the context of large size particles but would not be effective in the context of particles having sizes in the range of 1-2 mils. An optical system is also subjective, based on operator judgment and is relatively time-consuming.
Within the prior art, other techniques are known for determining imperfections which exist on surfaces. For example, IBM TDB, Vol. 14, No. 7, page 2130, December, 1971 checks for imperfections on magnetic recording discs by utilizing a thin copper film pad. This pad is placed on a checking head which moves systematically over the disc surface so that the copper rubs the surface. Projections on the surface will scratch the copper which can then be subsequently inspected utilizing an interference microscope. The profile of scratches obtained provides a measure of the size of surface imperfections. Such a system relies on optical checking and is relatively time-consuming since the entire surface must be scanned. The system also requires additional processing structure in the form of a checking head and off-line inspection. Techniques are also known to determine the planarity of workpieces such as the detection of oversize pinheads of a multiple pin carrier are disclosed in IBM TDM, Vol. 25, No. 5, page 1853, October, 1980. Pinhead planarity is detected in IBM TDM, Vol. 20, No. 8, page 3050, January, 1978.